When a vehicle encounters a disturbance such as a strong crosswind or side wind, and irregularities of the road surface, the vehicle tends to deviate from the intended path of travel. The steering system of a vehicle is often equipped with a power steering system which assists the manual steering input from the steering wheel with an output of an electric motor or a hydraulic motor. According to the conventional power steering system, the assisting torque is produced only when the vehicle operator has applied some steering torque to the steering wheel. Therefore, when the vehicle deviates from the intended straight path of travel due to crosswind, the electric or hydraulic motor will not produce any assisting torque unless the vehicle operator applies a steering torque which counteracts such a deviation.
Therefore, to prevent the deviation of the vehicle from the intended path, the vehicle operator must constantly apply a manual steering torque to the steering wheel. According to the conventional steering system, the assisting torque is generally small when the vehicle speed is high. Therefore, when the vehicle deviates from an intended path of travel, the steering input necessary to restore the vehicle to the intended path of travel becomes progressively greater as the travelling speed of the vehicle increases.
It was proposed in Japanese patent application No. 3-277023 corresponding to U.S. patent application Ser. No. 07/950,081, filed by the assignee of the present application to detect the irregular behavior of the vehicle caused by external disturbances from the yaw rate of the vehicle, and to produce a counteracting torque with the electric motor for power assist so as to control the behavior of the vehicle against the external disturbances. In other words, the counteracting torque is given as a product of a yaw rate and a control coefficient.
According to such a system, when the gain of the control system or the control coefficient is increased with the intention of enhancing the control action for a given yaw rate, if the vehicle operator releases the steering wheel, the angular movement of the steering wheel demonstrates a tendency to oscillate and overshoot. It is possible to control such oscillatory movement and overshooting of the steering wheel by applying a damping torque to the steering wheel which is given as a product of a damping coefficient and the angular speed of the steering wheel, and appropriately selecting damping coefficient. It is thus essential to properly select the gain of the system with respect to the yaw rate and the damping coefficient for the angular speed of the steering wheel if both a satisfactory control and a desirable attenuating property of the system are to be attained.
However, according the previously proposed control system, the gain for the yaw rate and the damping coefficient for the angular speed of the steering wheel had to be selected individually even though these two factors affect each other, and changing one of them would necessitate the readjustment of the other. For this reason, according to the previously proposed system, it was difficult to achieve both a satisfactory control and a desirable attenuating property of the system. It was also found by the inventors that the gain for the yaw rate must be changed according to the vehicle speed to ensure a satisfactory handling of the vehicle.